Balance and Stability

Stability Definition: the resistance to a change in the body’s acceleration, or the resistance to a disturbance of the body’s equilibrium Stability can be enhanced by determining the body’s centre of gravity and changing its position appropriately Definition: The ability to control equilibrium (either static or dynamic) Balance

Base of support (BOS) Definition: The supporting area beneath the body Includes the points of contact with the supporting surface and the area between them These points may be body parts (such as feet), or extensions of body parts (such as crutches or other walking aids) Application: Stand with your feet together and have someone push you from the side. Then spread your legs 3 feet apart and be pushed again. Which one is more stable?

At this moment who is more stable?

Are these athletes balanced?? Why?? Why not??

Centre of Gravity Can be defined as “the point at which all parts of an object are equally balanced”. When we stand upright the C.O.G. is around the naval area. C.O.G. changes depending upon the body position. C.O.G. can be outside the physical body

Center of gravity (COG) Definition: the point at which all of the body’s mass (weight) is equally balanced = center of mass (COM) Location of the COG depends on: The proportion of body parts The distribution of fat and muscle mass in the body Posture External forces (e.g., carrying a shoulder bag) COG changes position during movement: Lifting the arms raises COG Bending the knees lowers COG

Centre of Gravity This is the centre point of an object’s mass around which the rest of the object is equally balanced. The COG in a persons body is also called the balance point around which movement will pivot/ rotate. This is located about the height of the naval, unless the body changes position.

Main factors contributing to stability Position of the centre of gravity: –An object is in stable equilibrium when its centre of gravity lies within its base of support –The upright body is only stable when the line of gravity lies within the foot base Base of support (BOS): The upright body is least stable when the feet are parallel and close together (because the BOS is small) As the feet are moved further apart and the BOS increases, stability increases

Line of Gravity (LOG) Definition: Gravity’s action line which is visualized as a vertical line projecting downwards from the center of gravity Line of Gravity

Stability in carrying a weight at the side of the body Unstable Stability in carrying a weight at the side of the body Stable

Unstable = outside Stable = inside

Line of gravity Centre of gravity Base of support Line of gravity Centre of gravity Base of support

On the following pictures draw the BOS, COG, LOG

LOG, COG & Physical Activity The LOG must go outside the BOS to start or continue movement The direction the LOG from the BOS will be the direction the object will move in The further away the LOG from the BOS the greater the tendency to move in that direction

Factors for achieving balance A person has balance when the COG falls within the base of support (BOS) A person has balance in direct proportion to the size of the BOS (the larger the base of support, the more balance) A person has balance depending on mass (the greater the mass, the more balance) A person has balance depending on the height of the COG (the lower the COG, the more balance) A person has balance depending on where the COG is in relation to the base of support (balance is less if the COG is near the edge of the BOS)

Movement is towards LOG Direction of Movement Foot pushes towards the ground

Clinical applications If we bend to pick up a child or a box, the knees should be bent and the trunk flexed to move the COG down and over the feet In standing up from sitting, stability can be increased by moving the feet back and leaning the trunk forward in preparation for standing

Try This Out… Think about it… The lower the COG to the base of support the more stable. Stand with your feet one foot apart and have someone push you from the side. Then lower your CG closer to the base of support and be pushed again. Which one is more stable? The greater the mass of the object the more stable it is to an external force. A 450 pound sumo wrester is more stable than a 200 lb wrester of the same height. Which one is easier to knock over? Think about it…

3 ways to enhance stability Lower the COG Widen your BOS Ensure the line of gravity is within the BOS